High-Temperature Nonlinear Polyimides for Ï(2)

Chapter 10

High-Temperature Nonlinear Polyimides for χ Applications Downloaded by UNIV OF CALIFORNIA SANTA BARBARA on March 7, 2016 | http://pubs.acs.org Publication Date: August 11, 1995 | doi: 10.1021/bk-1995-0601.ch010

(2)

R. D. Miller, D. M. Burland, M. Jurich, V. Y. Lee, C. R. Moylan, R. J. Twieg, J. Thackara, T. Verbiest, W. Volksen, and C. A. Walsh IBM Almaden Research Center, 650 Harry Road, San Jose, CA 95120-6099 A variety of thermally stable N L O chromophores have been attached via a short tethers to 3,5-diaminophenol to produce NLO substituted aromatic diamine monomers suitable for polyimide formation. This may be accomplished either via nucleophilic displacement or by Mitsunobu coupling for base -sensitive materials. The functionalized diamines have been incorporated into a variety of polyimide materials yielding thermally stable polymers with Tg's ranging from 210-230°C. These materials may be poled at or near the polymer glass tem perature and the polar alignment thus induced was stable for long periods at 100°C. The improved thermal stability of the chromophores containing diarylamino donor substitution was also manifested in the cured polymers and is also apparently not significantly jeopardized by the inclusion of an alkyl tether.

Organic nonlinear optical (NLO) materials are of interest because of the large and fast intrinsic nonlinearities, high damage threshold limits and ease of processing {1,2). They have been studied in various forms, including: crys tals, organic glasses, Langmuir-Blodgett films, vapor deposited films, poled polymers, etc. For organic modulator and switch applications, poled polymers are the materials of choice. It is now apparent that fully integrated polymeric modulators and switches will operate continuously at elevated temperatures (80-100°C) and could experience brief excursions to 250°C or more for short periods during the various integration and packaging steps (3,4). This places additional demands on the device materials beyond just high nonlinearity. The elevated operating temperatures will require thermal stability for the N L O polymer; both orientational stability of the induced polar order and intrinsic chemical and thermal stability of the chromophore and polymer. Although much experimental effort has been expended to improve the orientational stability, somewhat less has been devoted to the actual development of thermally stable chromophores. 0097-6156/95/0601-0130$12.00A) © 1995 American Chemical Society Lindsay and Singer; Polymers for Second-Order Nonlinear Optics ACS Symposium Series; American Chemical Society: Washington, DC, 1995.

Downloaded by UNIV OF CALIFORNIA SANTA BARBARA on March 7, 2016 | http://pubs.acs.org Publication Date: August 11, 1995 | doi: 10.1021/bk-1995-0601.ch010

10.

M I L L E R E T A L .

High- Temperature Nonlinear Polyimides

131

Initially, orientational stability was sought using crosslinking techniques to restrict the chromophore rotational mobility (5). This approach has resulted in materials with large nonlinearities and impressive polar order sta bility, however, these systems are often difficult to process into operating devices due to embrittlement, cracking, adhesive breakdown, dimensional changes upon curing, etc. Although factors such as chromophore size, poling procedures, polymer aging, chromophore-polymer interactions, etc., can influence the relaxation rate of poled polymer systems, it is clear that the glass transition temperature (Tg) of the system plays a primary role (5,6). For this reason, thermoplastic materials with high glass temperatures have recently attracted considerable interest. This has included both host-guest composites (7-13) as well as polymers with chemically bonded chromophores (5,14-20). Furthermore, it has recently been suggested that a Tg which is 100°C or more above the highest continuous use temperature is necessary to maintain the polar order for long periods of time (21). This has driven the current quest for very high Tg NLO polymers. The high temperature poling process, coupled with the need for low optical losses and dimensional stability in a polymeric waveguide configuration, create a new set of difficulties to be addressed for high temper ature NLO systems. Thermally Stable NLO Chromophores Recent studies on NLO chromophore thermal stability have shown that the more usual and unfortunate relationship between chromophore nonlinearity and thermal lability is not inviolable (22-24). In fact, we have recently (17,21,22,25,26) demonstrated that the thermal stabilities of a variety of stilbene, tolane and azo derivatives can often be greatly improved simply by the substitution of diarylamino donor substituents for the more common dialkylamino derivatives without seriously compromising chromophore nonlinearity. This characteristic is apparent for a wide variety of electron accepting and electron transmitting functionality. The pertinent thermal and nonlinear data for a variety of nitro substituted chromophores is shown in Table I. The column headed nfl^ ( — co; co, 0) provides a comparison of the calculated hyperpolarizability for the electro-optic effect derived from /?(2co; a>, to) measured by electric field induced second harmonic generation (EFISH, 1.907 pm) extrapolated to 1.3/im times the measured molecular dipole moment in Debye (27). This is a particularly pertinent quantity for electro-optic poled polymer applications in the infrared. A related quantity (f*P\j( —to\co, 0)/MW) represents a crude attempt to scale the poled molec ular nonlinearities to molecular size (approximated here by molecular weight). This quantity, which we define as the reduced molecular nonlinearity, will be relevant to the macroscopic bulk nonlinearity which depends on the number density (molecules/cm ) of the NLO chromophore (1.2). The onset decom position temperature, designated as (21) in Table I, is derived from differ ential scanning calorimetry (DSC) data run at a heating rate of 20°/minute. This number, when compared with use temperatures derived from a complete three-part thermal analysis (21), which we have developed for the assessment of the thermal stabilities of potential NLO chromophores, provides only an upper limit. In practice, temperatures often as much as 60 -80°C lower than Tjj, as determined by DSC, may be more appropriate for estimating the 3

3

Lindsay and Singer; Polymers for Second-Order Nonlinear Optics ACS Symposium Series; American Chemical Society: Washington, DC, 1995.


132

POLYMERS FOR SECOND-ORDER NONLINEAR OPTICS

Table I. The thermal, linear and nonlinear properties of a variety of nitro substituted NLO chromophores: a. absorption maximum in chloroform; b. quadratic molecular hyperpolarizability extrapolated to zero frequency assuming the validity of the two-state model (5); c. values of the molec ular nonlinearity for the electro-optic effect extrapolated to 1.3 fim x the dipole moment (D) (25); d. calculated values for the reduced nonlinear ities extrapolated to 1.3 pm. c A

Structure

TdCC)

Ph N ^ 0 - CH=CH - ^ s ^ O L 2

Ph N^-CH=CH^]L 2

N 0 2

Ph N - @ - CH=CH

N0

2

™

H

2

2

N - C M - C H o

N

2

max

(nm)

/*(D)

A,(«") X10

b

flfi

13

(-M.fi).0)

30

d ^1-3 MW

x 1 0

356

550

7.21

71.8

1400

3.2

298

582

6.89

68.2

1300

3.2

367

458

5.48

57.2

757

1.7

325

492

5.18

56.8

738

1.9

393

486

5.87

54.3

794

2.0

358

436

4.75

37.3

419

1.1

336

418

4.84

28.2

317

0.9

303

410

6.57

27.1

345

1.4

O

2

N - © - N = N - © - N 0

2

8 (D03)


30

10.

M I L L E R

E T A L .

High-Temperature Nonlinear Polyimides

133

thermal robustness of NLO chromophores for poled polymer applications. For a variety of substituted chromophores, the data in Table I and reference 22 show the efficacy of diarylamino donor substitution. We have somewhat arbitrarily targeted chromophores with reduced nonlinearities > 2.0x10 '"era -D/esu and T temperatures > 350°C for further study. The thermal analysis data recorded at scan rates of 20°/minute, while not predictive in an absolute sense, are useful in a relative sense for comparing prospective chromophores. In general, we find that the thermal stability of the nitro substituted azobenzene derivatives and stilbenes are often compa rable, although exceptions have been observed (22). These, in turn, are both more stable than the corresponding tolane derivatives. Similarly, the molec ular nonlinearities for the azo and stilbene derivatives are also often compa rable. For these reasons, as well as for synthetic efficacy, the azo derivatives were selected over the related stilbenes for detailed study.


d

Synthesis In principle, the most straightforward route to N,N-diarylaminophenylazobenzene derivatives would be the direct azo coupling of a substituted benzene diazonium salt with an appropriately substituted triphenylamine derivative (28). A phase transfer variant of this procedure, in fact, works for the preparation of the parent 4-N,N-diphenylamino-4'-nitroazobenzene 5, albeit in moderate yield (47%) (29). However, this technique fails completely for substituted triarylamino derivatives containing strongly electron donating substituents. The problem is illustrated below for N-p-methoxyphenyldiphenylamine 9. In this particular case, a complex mixture results from attempted azo coupling, from which the dearylated azo dye DOl [11] is iso lated as the major reaction product even when run under phase transfer con ditions. Similar problems resulted with triarylamine derivatives substituted with other electron donating substituents such as amino, acetamido, trifluoroacetamido, etc. This is unfortunate, since such substituents are useful for the attachment of tether functionality for ultimate incorporation into a polymer (vide infra). We have found, however, that a variety of diarylamino substituted azobenzene derivatives may be prepared directly from preformed, commer cially available azo dyes by a variation of the Ullmann coupling procedure using cationic phase transfer polyethers (30). The reaction itself and some pertinent examples are shown in Table II. In the case of 4-N,N-diphenylamino-4-nitroazobenzene (5, entry 1), the isolated yield of the product by Ullmann coupling was almost twice that obtained by the diazonium coupling route (81 vs. 47%). A wide variety of aryl substituted azo dyes can be gener ated in this manner, as shown in Table II. We have routinely employed two synthetic variants of this procedure. Method A uses a solid-liquid phase transfer procedure (31) utilizing 18-crown-6 and anhydrous potassium carbonate in a high boiling solvent such as o-dichlorobenzene. Reaction tem peratures of 170-180°C and run times of 5-14 hours are typical. After completion of the reaction, the solvent is removed under vacuum and the residue purified by flash column chromatography (32). A second procedure (Method B) utilizing triethylene glycol dimethyl ether as both the solvent and phase transfer medium and higher reaction temperatures (205-210°C) for 2-6 hours has been used in a few cases. Using this procedure, the reaction ,



134


Table II. Preparation of aryl substituted azo dyes by modified Ullmann coupling. Procedure A: solid-liquid phase transfer (based on 1 mmol of the azo starting material); 4 mmol Cu, 0.3 mmol l8-crown-6, 2.5 mmol K C 0 , o-dichlorobenzene, 185°C, 12 — 18h; Procedure B: 4 mmol Cu, 2.5 mmol K C 0 , triethylene glycol dimethylether, 210°C, 2-5h. 2

3

2

3

H I ___

r 1

1

-m-0~ " n

n H

N0

2

Ar . I R-N - © - N = N - ^ - N 0

+ Arl

2

Entry

R

1

Ph D01(11)

81

144-6

486

2

Ph

68

154-5

498

3

Ph

74

171-3

486

Aryl Halld*

Y1..d(%)

•-©-• t-BuPh SI0—i 2

4

Ph

t-BuPh SK> N ^ \ _ ^ ^ _ 2

0

"

NH

Ph

HN- @ -

Ph

8

H D03(8)

b

SO

496

2

P

M«0

N0

2

56

169

496

61

155-7

498

55

116

492

40

168

510

19

210-12

448

40

255

502

IB

M

2

M«0 7

N-N

N

/ Ph 6

Ph Ph

~*

1*

>A

Ph

3

^ 0 ^ 0 ) - N - © - N = N ^ - N 0 0

0 5

1

mpCC)

N-N

N0

2

h

^ - N - ^ - N - N ^ © - N 0

2

0 0 9

H

>^NH-^-I

>^^NH

NH-@-

N=N - 0 -

N0

2

0

(^NHH0^NH©-N-NH^-NO

2



10.

MILLER ETAL.


135

is monitored by TLC, and when complete, the mixture isfiltered,diluted with water and extracted with ethyl acetate. Examination of Table II shows that the synthetic technique is quite general. By controlling reagent stoichiometry, one can achieve monoarylation with bifunctional reagents (entry 3), arylation with reactively functionalized iodides (entries 5, 6 and 9) and one pot bisarylation of primary amines (entries 8 and 9). The direct production of 4-N,N-diarylamino-4'-nitroazobenzenes containing a flexible tether group (entry 4) is particularly pertinent, as the silicon protecting group is easily removed with 1M tetrabutylammonium fluoride in THF and the resulting alcohol functionality can be used to generate polymerizable NLO monomers (vide infra). Attempts to utilize aryl iodides containing unprotected alcohol functionality directly in the Ullmann coupling were unsuccessful. Since the ultimate goal of this work was the preparation of polyimide derivatives containing thermally stable chromophores, a simple route to NLO functionalized aromatic diamines was developed which is shown in Scheme 1. The alcohol 20a is the commercially available azo dye DR1. The diphenylamino substituted derivative 20b was prepared as described in Refer ence 33. The functionalized aromatic diamines M l and M2 were prepared by nucleophilic displacement from the corresponding NLO functionalized tosylates 21a and 21b and subsequent hydrolysis. Under strongly basic con ditions, the alkylation reaction takes place chemoselectively, primarily on oxygen. The bifunctional monomer M3 could also be prepared in 53% overall yield using the same procedure starting from 3,5'-dihydroxy4,4'-diaminobiphenyl (23). Although this procedure works well in some cases, it fails completely for a number of important examples. For example, attempted displacement from the tosylate derivative of 24 led to immediate decoloration of the solution. The starting 6-nitrobenzothiazole substituted alcohol starting material 24 was prepared by azo coupling of 6-nitrobenzothiazole-2-diazonium tetrafluoroborate 25 to 4-N-(4-hydroxyethylphenyl) diphcnylaminc 26. The latter was prepared by the Ullmann coupling of diphenylamine with t-butyldiphenylsilyl protected p-(/J-hydroxyethyl)iodobenzene followed by treatment with 1M Bu NF in THF. Similarly, the reaction also failed for a variety of other chromophores containing base-sensitive tricyanovinyl acceptor groups. For these thermally stable, but reagent-sensitive chromophores, another procedure was developed utilizing the Mitsunobu (34) coupling reaction shown in Scheme 2 (17,35). Since the reaction failed using the free amine, the amino substituents werefirstselectively protected as trifluoroacetamides. The pro tecting groups could be easily removed after coupling by treatment with weak base. Using this procedure, the overall yield of M4 from 24 was ~ 50%. The yield of M2 could also be improved from 27% (Scheme la) to 60% using this procedure. The N-t-butoxycarbonyl protecting group can also be utilized when the functionality is sensitive to basic hydrolysis (as is observed for chromophores containing tricyanovinyl substituents) (35). In these cases, deprotection is accomplished using trifluoroacetic acid in methylene chloride at room temperature. In summary, using the procedures described, the NLO functionalized aromatic diamine monomers M l - M 4 could be prepared and purified. /

4


136



N L O Functionalized Polyimide Derivatives The diamines M l - M 4 could be converted into a variety of polyimide deriva tives using standard condensation polymerization procedures. In a dry box, equimolar quantities of the diamines and a suitable bis-anhydride (for example, either oxydiphthalic anhydride (ODPA) 30 or hexafluoroisopropylidenediphthalic anhydride (6 FDA) 31 were mixed in dry N-methylpyrrolidinone (NMP) and stirred at room temperature overnight to produce homogeneous solutions of the amic acids PA1, PA2, PA4 (Scheme 3). The corresponding amic acids derived from M3 could be prepared in a similar fashion. Amic acid films were prepared by dilution of the N M P sol utions with 15-20% o-xylene and spin casting. The spun films could be thermally imidized by heating slowly from 150-250°C. Alternatively, the amic acids in N M P could be chemically imidized by heating with acetic anhydride-pyridine. The polyimides thus formed were isolated by precipi tation into water or water-methanol followed by filtration, washing and vacuum drying. The polyimide derivatives derived from ODPA (PI-2a, PI-3) are insoluble in common solvents and films were prepared by casting the amic acids followed by thermal curing. The hexafluoroisopropylidene substituted polymers (PI-1, PI-2b, PI-4), however, were readily soluble in solvents such as cyclohexanone, 1,1,2,2-tetrachloroethane, cyclopentanone, etc., and films were prepared by spin casting. The solvent-spun polyimide films were proc essed by heating in a stepwise fashion at 150, 200, 250°C for 1/2 h under N and finally to 275°C for 15 minutes. The Tg values of the polymers PI-1 to PI-4 ranged from 210-230°C, as measured by DSC analysis at a heating rate of 20°/minute. The long wavelength visible absorption maxima of the N L O chromophores in PI-2a,b, PI-3 appeared around 500 nm while that of PI-1, which contains a DR1 subunit, absorbed at shorter wavelengths (~ 474 nm). The benzothiazole derivative PI-4 was more purple in color and absorbed at 548 nm. The T G A scans of the cured polyimides P M - P I - 4 showed no significant weight loss below 350°C. Comparative DSC analysis of PI-1 and PI-2b show clearly the improved thermal stability of the latter. For these particular examples, the corresponding onset decomposition temper atures (T ) were 338°C and 376°C respectively (see Figure 1). It is inter esting to note that the incorporation of an alkyl tether group does not seem to effect the thermal stability of the chromophore in the polymer very much (27). The thermal stability of the cast and processed polyimide derivatives was also studied by variable temperature UV-visible spectroscopy. For these studies, relatively thin films (0.5-0.7 fjm) were utilized and the change in the optical density at the long wavelength maximum was monitored upon heating. Figure 2 shows the direct comparison between two 6F-polyimides PI-1 and PI-2b. PI-1 contains a dialkylamino substituted chromophore while the PI-2b has diarylamino donor substitution. The former is reasonably stable to heating under N at 250°C, but decomposes rapidly at 275°C. PI-2b is completely stable at 275°C and is still reasonably stable at 300°C. Above 300°C, all of the polyimides described here show some signs of decom position, as evidenced by changes in their UV-visible spectra. The 6-nitrobenzothiazole substituted polymer (PI-4) was only slightly less stable than the other diarylamino substituted derivatives and lost ~ 8% of the ori ginal absorbance at 548 nm upon heating at 275°C for 95 minutes. The fact 2

d

2



10.

137


M I L L E R E T A L .

@ - N H - 0 - N =

N

- Q > - N O

NO,

10

11(001)

Ph

0,N 24

OR o

0

OR

Ph PI-3

X = 0. R =

— (CH 2 ) 2 0

- © - N - @ - N = N - @ - N 0


2

2

138


R

R

1


R -N^®*-N=N^®-N0 20a

R'=

— (CH ) 0H.

20b

R

- ^ - 0 - ( C H

1

2

-

R

2

)

2

2

2

1

R -N-®-N=N-@-N0

2

= Et

0 H .

R

=

2

Ph

21a

R

1

•= — ( C H ) 0 T « . R * = E t

21b

R

1

«= - ^ - O - C C H ^ j O T t . R * =

2

2

2

Ph

HN 2

H

*

N

•12 2

2

H.N

b,c HN 2

y 1

R* • —

(CH )

2

M2

f?=

( C H

2

—

2

NEt

)

2

N=N

N 0

(54%)

2

0 - ^ - N - ^ - N = N - ^ - N 0

(27%)

2

Ph a.

p-toluen«sulfonyl chloride, triethylamlne, dlmethylamlnopyridlns C H C I , 40 °C; b. NaH-THF, 25 °C, C. NMP, 70 C, 1.5h.

(DMAP)

W

2

2

0R

3

2

1

b

+ H

2

N - ^ 0 - N H

HO

^ — •

2

H

OH

2

N - 0 - 0 - N H

2

0R

3

23 MS

R

3

= » - ( C H

2

)

2

0 - ^ - N - ^ - N = N - ^ - N 0

2

(53%)

Ph

Scheme 1. The preparation of NLO functionalized aromatic diamines by nucleophilic substitution.



Q

b

P h

>

24

CF C0NH' 3

2

3

27

«^OSlPh t-Bu

CFjCONHv

.

q

•

^ ^ - °

*

3

2

CF CONHv

28

CFJCONH'

3

CF C0NHV H

P h

29

4

2

3

2

Scheme 2. The preparation of NLO functionalized aromatic diamines by the Mitsunobu reaction coupled with selective deprotcction.

3

c. B u NF-THF(1M), 25°C, 1h; d. Dllsopropylazodlcarboxylate, THF, Ph P; e. K C 0 , MeOH-H 0 (3:2).

3

a. t-BuPhjSICI, trlethylamlne. CH CN, reflux. 2h; b. (CF CO) 0. pyridine, A reflux. 2h;

28

2

22

tt^-OH

H N'

2

n Nv


140


that chromophore decomposition as determined by variable temperature UV-Visible studies occurs at lower temperatures than predicted by DSC may be attributed to the heating rate dependence of the data obtained by the latter technique (21). Nonlinearity Measurements The electro-optic coefficients (r ) for the polymers PI-1 through PI-4 were measured by one of three procedures depending on the poling technique, and the results are shown in Table III. For samples that were poled by corona discharge, an attenuated total reflection (ATR) technique was employed (36). When single films were electrode poled, an ellipsometry technique was utilized (37,38). A heterodyne technique (39,40) was used to determine r of polymers poled in modulator geometries. With the second method, rather thin films (NLO polymer layer only) were utilized (1.5-2.5 fim) and the poling field was maintained at a relatively low and constant value (~75V///m) to avoid any possibility of shorting and damage to the film. The values of the electro-optic coefficient (r ) obtained in these electrode poling experiments would be expected to increase linearly with field (5). In the case of PI-2b, a triple-stack phase modulator composed of two crosslinked acrylate buffer layers sandwiching the N L O polymer layer was constructed and larger poling fields were applied ( ~ 250 V//*m). The r value measured in this modulator was similar to those obtained both by corona poling and by single polymer layer ellipsometry upon extrapolating the latter to the modulator poling voltage. In the case of two related N L O polyimides, the r values measured either at 250 V//xm and/or extrapolated to this poling field compared quite favorably with those predicted by calculation (27) (i.e., 8.8 pm/V for PI-2a and 8.3 pm/V for PI-2b, respectively). However, the measured value for the 6-nitrobenzothiazole polymer PI-4 seems anomalously low based on the substantially larger hyperpolarizability of the chromophore (22). Rough calculations of the r value predicted for this polymer indicated an expected value of around 20 pm/V at 250 V///m; considerably larger than the initial experimental value. It is possible that the poling is less efficient for this more extended chromophore. Greatly improved thermal orientational stability for the polyimides PI-1 - PI-4 would be anticipated by virtue of their relatively high glass transi tion temperatures (see Table III). Relaxation data obtained by monitoring Hie intensity of the second harmonic signal of films of PI-2a and PI-2b poled in a corona field (~220V/j*m, poling temperatures 210-220°C) are shown in Figure 3. After a small decay in the signal of - 5%, each sample was stable for over lOOOh at 100°C. In addition, preliminary results suggest that annealing the sample of PI-2b at 175°C for ~ 3h in the poling field increases the characteristic relaxation time T (derived from a stretched exponential fit (5) of the decay data) by - 50%.


33

3 3

33

3 3

3 3

3 3

Conclusions In summary, we have described the preparation of a number of thermally stable diarylamino substituted nitroazobenzene derivatives by a modified Ullmann coupling procedure using commercial azo dyes. The procedure appears quite general and is applicable to materials containing potentially reactive tether functionality for formation of N L O functionalized aromatic



10.

MILLER ETAL.

141


OR -NH

HMN

30

X =

31

X =

0 C(CF ) 2

Ml

R = - f C H

M2

R

2

)

NEt

2

H0 C 9

NHM

- ® - N = N - @ - N 0

= - < C H

2

)

O - ^ - N - 0 - N = N

2

X = CfcFjk

PA-1

2

2

PA-20 X = 0 PA-2b X = C(CF )

2

PA-4 X = C(CF,)

2

S

Ph M4

R

= - < C H

2

)

2

- Q - N NO.

b or e

OR PI-1

X =

C(CF ) .

M

X =

i c F

S

3

)

R =

2

2

}

R

—

=

-
»

°

^

- ^ - N = N - ^ - N 0

©

- N

-

©

^

N

=

N

2

^

N

0

2

Ph PI-4

X =

C(CF ) S

2

R = - ( C H

2

)

2

N - ^ - N = N

Ph 0 . NMP. 25 " C , 12h. b. A. 250 " C .

c. A c 0 2

NO.

pyridine

Scheme 3. The preparation of NLO functionalized side chain polyimides by condensation polymerization.


142

P O L Y M E R S

F O R S E C O N D - O R D E R

N O N L I N E A R

O P T I C S

II

8

OR


Ph

2 Td=338*C

—

0 I 0

II

I I I I | I I I I

50

I I I | I I I I | I I I I | I I I I | I I M

100

150

200

250

300

| I I M

350

| M

I I | M

I I | I*

400

450

500

Temperature (°C)

Figure 1. DSC analysis of the polyimides PI-1 and PI-2b measured at a heating rate of 20°/minute.

\

\

0

10

\

\

i

i

i

i

20 30 40 50 60 70

—

i

80

—

i

—

i

—

90 100

Time (minutes)

Figure 2. Variable temperature UV-Visible studies of films of PI-1 and PI-2b. Absorbance measurements at the A „ of the long wavelength visible transition (474 nm for PI-1 and 498 nmforPI-2b). ma


10.

MILLER ET AL.

143


Table III. The thermal, linear and nonlinear optical properties of a variety of NLO functionalized side chain polyimides: a. poling temper ature; b. polymer glass transition temperature as measured by DSC anal ysis at a heating rate of 20°/minute; c. electrode poling (75 V///m), r value measured by ellipsometry; d. corona poling (~ 230 V//im), r measured at 1.3//m by attenuated total reflection (ATR); c. corona poling (200 V//zm); f. measured in a stacked thrcc-lcvcl waveguide phase modulator; g. corona poling (230 V/jxm), r measured by ATR at 633 nm and extrapolated to 1.3 /im assuming the validity of the two-state model (5). 33

33


3 3

b

PI-1 PI-2a

Tpol T (°c) 'g (°c) 205 213 210 223

PI-2b

220

0

Polyimide

228

^max (nm)

Chromophore Wt. %

474 500

36 47

498

45

r (1.3/xm) (pm/V) c 3.75 8.2-10.0 33

f

8.1 L L 3.1 J 13.0 c 2.45 C

PI-3 PI-4

210 220

210 225

496 548

9

58 53

1.0 —___

XT W

0.9

V

v

§0.8 §0.7

PI-2a T =223C -

g

0.6 0.5 1.0

(a) I

I

J

L_

L_

0.9 g 0.8 §0.7

PI-2b T=228*C lf

0.6 (b)

0.5

-j

i

200

i

i_

400 600 800 Time (hours)

1000

Figure 3. Decay in the second harmonic coefficient at 100°C, laser fun damental 1.047 firm a. PI-2a, Tg = 223°C; b. Pl-2b, Tg = 228. Lindsay and Singer; Polymers for Second-Order Nonlinear Optics ACS Symposium Series; American Chemical Society: Washington, DC, 1995.


144


diamines. At the same time, a chromophore containing the intrinsically more nonlinear 6-nitrobenzothiazole unit with a functionalized tether was prepared by direct azo coupling. The chromophores containing hydroxyalkyl or hydroalkoxy tethers could be connected to 3,5-diaminophenol via an ether linkage either by direct nucleophilic displacement or by the Mitsunobu cou pling of the alcohol. The latter procedure is quite general and allows the preparation of thermally stable, but reagent sensitive, monomers which cannot be coupled by nucleophilic displacement under basic conditions. The N L O functionalized diamines can then be incorporated into polyimides by standard condensation polymerization techniques. Polyimide films can be prepared either via the functionalized polyamic acids by thermal curing or from the preformed polyimides themselves, when soluble. These techniques produced a variety of functionalized polyimide derivatives with glass transi tion temperatures ranging from 210-230°C. The improved thermal stability of chromophores containing diarylamino relative to dialkylamino substitution was demonstrated both by DSC analysis and by variable temperature UV-visible studies. The polymers could be poled as the polyamic acid precur sors and as polyimides using either corona or electrode poling. In all but one case, the measured electro-optic coefficients were consistent with the approxi mate values calculated based on EFISH hyperpolarizability measurements. The thermal stability of the polar order of typical corona poled samples was excellent, showing a decrease in the second harmonic intensity of less than 10% over lOOOh at 100°C. The stability could be further improved by aging at elevated temperatures. These preliminary results suggest that by varying the N L O chromophore and/or the dianhydride, one could anticipate the prep aration of N L O polymers with both improved nonlinearities and elevated glass temperatures. Acknowledgments The authors gratefully acknowledge partial funding support from the AFOSR (F49620-92-C-0025) and the N 1ST-ATP program (Cooperative Agreement No. 70NANB2H1246) for this project. One of us (T.V.) would like to acknowledge support from the Commission for Educational Exchange between the USA, Belgium and Luxembourg, The Belgian National Fund for Scientific Research (NFWO) and from IBM Belgium. Literature Cited 1. 2. 3. 4. 5. 6.

Nonlinear Optical Properties of Organic Molecules and Crystals; Chemla, D . S., Zyss, J., Eds.; Academic Press: New York, 1987. Prasad, P. N.; Williams, D . J. Introduction to Nonlinear Effects in Monomers and Polymers; John Wiley & Sons: New York, 1991. Boyd, G . T. In Polymers for Electronic and Photonic Applications; Wong, C . P., Ed.; Academic Press: New York, 1993; p. 467. Lipscomb, G. F . ; Lytel, R . Mol. Cryst. Liq. Cryst. Sci. Technol. B, Nonlinear Optics 1992, 3, 41. Burland, D . M.; Miller, R. D.; Walsh, C . A. Chem. Rev. 1994, 94, 31 and references cited therein. M a n , H . T.; Chiang, K . ; Hass, D . ; Teng, C . C.; Yoon, H . N. Proc. SPIE 1990, 1213, 77.


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